The Mass Graves, southwest of Garren's Haunt to the north, were made to accommodate the...impressive...number of deaths Tirisfal suffered when the Plague first came.
The bodies in these graves have so far been spared an undeath, but the Scourge now send Rot Hide Gnolls to gather the corpses and use them to bolster their armies. This cannot be allowed!
Your task is twofold: slay the Rot Hides at the Mass Grave and Garren's Haunt, and gather from them the Embalming Ichor that gives them life.

Expertly done, <name>. The Scourge are mistaken if they think they can use those corpses against us, and the fluid you gathered from the Rot Hide slaves will be studied by our Apothecaries. It may have secrets we can use against them.
As I said, well done. But our struggle continues, and the conflict will surely offer you more chances to prove your worth to the Forsaken.